As stated in the long title...

The thing is, my former colleague, say John, is applying for a postdoc with boss A in research area A at institute A, which is what he did for his PhD. Boss A replied, asked for some details and reference letters, as his colleague who also worked in area A, I am on his referee list. Last week I received an email asking for a reference letter for John.

However, the email is not from boss A, it's from boss B in research area B at institute A. This boss B asked specifically that I write a letter for John in area B.

Although research B is somehow related to research A, John's strongest experiences are definitely NOT in area B. Me and John are both confused. We are guessing, boss A was interested in John but doesn't need a postdoc right now so he recommended John to boss B. B needs a postdoc (very much) now, but in area B.

It gives me a headache, It won't sound like a reference letter if I just say that John is experienced in area A, not in B but he can learn fast etc... But if I just say how brilliant John is in both A and B, it would be lying and may give boss B false hope, it'd be also hard for John if he actually get the position.

The truth is, John is a good researcher, no doubt of that, and he has a lot of knowledge (not experience) in area B. It will take him some time to learn new things in area B, this time will be longer than an expert in area B but will certainly be much shorter than PhDs or who are not specialised in either A or B.

What should I say in this letter? to make John sound good enough for this position and also make it clear that boss B shouldn't have too high expectations for John...

Any suggestions? the sooner the better :)

Cheers!

Sorry if the above sounds complicated with all the As and Bs....

In short, John applied for a postdoc position but get a research proposal with a different boss in a different research area. This boss has never talked to John. Now he contact John's referee -- me, for a reference letter in this different area, where John isn't very familiar with.
What should I say in the reference letter?

Thank you all in advance!

Maybe you should talk about him and his scientific career in general without specifying any field. Like he has x papers published. He had this contribution to group meetings, or college..

I agree, if he's interested he probably knows the person doesn't have experience in that area anyway. Just keep it general and talk about other skills that are transferable.

Hey, Mathcomp, Treeoflife, thank you so much for the reply!
Keep it general is probably the safest way...
Maybe it's just the words in the email that got me worried, it's very specific about the topic, the boss even gave a long specific title for me to write... It makes me feel that, what he was asking was "will you recommend this guy in this particular area?"... Somehow I feel I'm responsible for this....
Maybe I'm just paranoid....

Of course it is not your job! You are only doing a favour to boss and to John. Actually keeping general is a polite way of saying "Hey boss, I don't know! you should find it out in another way". I'm sure your friend does not expect you to tell lie. Just say all general good thing you know about him and ignore any question bossed asked and you don't want to reply. Good luck!