Are you sure this is not a case of the #1 most common programming mistake that we see on the forum ?
Which basically means in your case: are you sure that you are allocating a circular buffer FOR EVERY channel that you have, and that, for every one of those channel, you are writing and reading samples from the corresponding buffer?
Because if you are dealing with two channels, and you process the first (reading and writing from a circular buffer), and then process the second (by reading and writing from the SAME circular buffer used for the first channel), that’s probably NOT what you want.
Other from that, isn’t FL Studio available for a limited time evaluation? If so, you can start the demo of FL Studio and hopefully reproduce then debug the issue your friend is seeing.
If it is not, you can contact its manufacturer directly and ask for a Not For Resale copy of FL Studio for the purpose of testing plug-in compatibility