Back in 2000 I published an article in the journal Criminal Law Forum, entitled "Human Rights and International Mutual Legal Assistance: Resolving the Tension." The cite is (2000) 11 Criminal Law Forum 143, but you can read a pre-publication version here. "Mutual legal assistance" (MLA) essentially refers to states gathering/seizing evidence on their territories and sending it to treaty partner states, for use in criminal proceedings being conducted in the partner states. In the article I examined a number of issues about the human rights implications of states providing MLA to other states with which they are cooperating, and whether and in what circumstances doing so would violate the international human rights law obligations of the requested states. While it was fairly well-established that states parties to human rights treaties (particularly the ECHR) would breach their obligations by extraditing individuals to face human rights abuses in the requesting state, the question was much more diffuse when it came to MLA because the general view was that MLA did not engage the rights of the individual more closely. Thus, states tended to reserve the right to provide MLA in such circumstances, though as a matter of policy many human rights-promoting states tended to avoid it.
In putting together the article I was lucky enough to be able to interview (in 1998) an official from the Edinburgh office of the UK's Crown prosecutions office, who told me that the UK's policy matched the description above -- as a matter of policy, the UK would not provide MLA to states where there were grounds to believe the accused would suffer human rights abuses, even though this might breach the MLA treaty ("MLAT"). This policy was, and has been in the years since, most keenly applied in death penalty cases. In 2011 the UK Foreign Office issued a policy document entitled "Overseas Security and Justice Assistance: Human Rights Guidance," which made clear that officials involved in international cooperation should seek assurances from the requesting state that the death penalty will not be imposed, prior to providing MLA.
(The most recent update of the policy can be found here; see also an excellent article by Bharat Malkani in (2013) 62 ICLQ 523).
Currently this policy is being litigated in the UK, in the case of Alexanda Kotey and El Shafee Elsheikh, two former UK nationals who fought with ISIS in the Syrian war, were captured by Kurdish forces, and are now facing prosecution for various crimes in the US. The UK Home Secretary initially indicated that the provision of MLA to the US in the case would have to suspended while the death penalty was on the table, but has since rescinded that decision and decided to provide cooperation, given the "exceptional circumstances" of the case and in particular the seriousness of the crimes. Elsheikh's mother has brought a legal challenge to this decision; coverage here and here. The argument being made is that the UK is legally obliged to comply with the policy.
A good account of the case and the applicable UK law by Prof. Malkani can be found here. The claimants have a tough row to hoe, not made easier by the fact that the two individuals in question were stripped of their UK nationality in 2014. Prof. Malkani makes the point, which I think is correct, that the law as it stands actually supports the government's position; and if one is inclined to think that states (particularly non-death penalty states) should be restrained from helping other states carry out the death penalty, then what the case points to is the need to strengthen the law.
My own observation is that, as I have remarked in conference papers from time to time, the international legal situation has not changed since my article was published in 2000. One of the pernicious problems of inter-state criminal cooperation is that states tend to resist the application of human rights norms to the process, and have to be dragged kicking and screaming into agreement with some fairly simple ideas, such as that to enable foreign states to commit human rights abuses makes the requested states complicit in those abuses. Those states which even care about this nonetheless maintain their practice at the level of policy, and resist both compulsion to do so by courts and interpretations of international and domestic human rights instruments that might lead in that direction.
The result? A still-diffuse set of international practices and norms around whether and how inter-state criminal cooperation should be shaped by human rights standards, starved for legal vitality by deliberate stickhandling on the part of states.