Yes, you're both right. Polyurethane (modelboards, tooling boards etc.) should NOT be used as a pattern or mould for prepregs. The reason for this is because - at elevated temperature - polyurethane inhibits the cure of epoxy-based prepregs. We have experience of this and the effect is really quite severe.

Instead, to make mould for prepreg you have a few options. If you want something that you can CNC machine then you should using epoxy tooling board, such as our EP700 epoxy tooling board. If instead you want to be able to laminate the mould (rather than machine it) then the following combinations make good options that are compatible with prepregs:

EG160 High Temp Tooling Gelcoat [--backed up with--] EMP160 Epoxy Moulding Paste
EG160 High Temp Tooling Gelcoat [--backed up with--] EL160 High Temp Epoxy Laminating Resin + woven glass fabric / or woven carbon fabric

Finally, I guess if you specifically wanted to be working with a foam (rather than a higher density board or laminating a mould) then you could use a foam like Rohacell (which is a PMI foam). We don't currently sell Rohacell but it is a foam you can use alongside prepregs without any inhibition or operating temperature problems.

I hope this helps.