Like with all UK visa applications, your friend will need to prove that he is intending to truly visit the UK short-term, that he can support that visit, and that he will leave after.
I assume from your question that your friend obtained the D-visa, probably 'scientifique-chercheur' or 'passeport talent', entered France, went to his OFII appointment, got the sticker, and is now applying for his first actual 'titre de sejour' - that is what the récépissé is valid for.
In this case, your friend should submit the UK visa application with his D-visa, his OFII sticker, and his récépissé. The récépissé is only valid alongside these other two documents. Whilst these are French documents, I'd imagine that the UK has probably seen them before. Still, I would include a note to the ECO to explain this, in the case of doubt. If your friend can afford to wait for his official carte de sejour, that may simplify matters slightly - but it would still be prudent to include his original visa documents, as that will show the ECO that he is abiding by French immigration law by entering when he should have and being lawfully in France during the entire duration.
Just as importantly, your friend would need to prove their intention to leave the UK at the end of the visit. I assume that your friend has a récépissé because he is 1 year into his 3 year contract, so he has 2 years of gainful employment left. Your friend needs to prove this to the ECO somehow. Your friend should provide a copy of their contrat doctoral, and some document showing that they have a 3 year (standard) French PhD contract (with 2 years left). A letter from his supervisor (saying that he is doing a PhD at lab X and has 2 years left) would probably help, as IIRC, the contrat doctoral is a 1-year thing which 'rolls over' and would not be proof in and of itself.
If your friend has a récépissé because his PhD is being extended by N months and his visa is being extended as well, I would not visit the UK at this time. This situation may raise suspicion that your friend might go underground. But based on what you described, it doesn't sound like this is the case, and your friend has just finished his 1st year.
Finally, the payslips for the past few months should prove to the ECO, alongside bank statements, that your friend's situation is as he claims.
Your friend should, of course, have these documents translated. I do not know whether he will need to use a sworn translator (this might be costly) or not.