What's your budget? because the best way is a hardware converter...Terranex. And if it's a file, they'll need to play out from one computer and capture in another.
Can try Compressor, export a small chunk, the one with the most movement. Change the frame rate, turn on the FRAME CONTROLS and set everything to BEST.
Still, this won't be smooth. 24p to 30p is not a good conversion, so 25p to 30p won't really be either. No way to spread out the repeated frames amongst fields. 25p to 30i would be the best way. Might try that. Even though it's 30i...it'll still look progressive. This is how films shot at 24fps are telecined to 30i tape, and shown on TV, that broadcasts at 30i...3:2 pulldown. Might get trickier with PAL...25fps.