Sounds a bit odd, is your friend sure it was ATC communications that he was picking up rather than something else?
I suppose it is possible, both FM radio and ATC R/T sit within a similar area of the VHF spectrum. FM radio can go up to about 108 MHz and I believe aviation R/T frequencies start at around 117 MHz (with radio nav aids filling in the gap between the two)... but for obvious reasons the specific frequencies used in different areas are picked very precisely to avoid any interference - broadcast spilling into avaition frequencies could be potentially disasterous, and aviation spilling into broadcast would see some seriously pissed off comercial radio licensees.
I guess in particularly freakish meteorological conditions it might be possible though.
Is bleed "restricted" and should he be making people aware?
If it happens regularly then it may be worth whinging to the BBC and other affected broadcasters, if it's a serious problem they can take it up with Ofcom who regulate the bandwidth allocation and may need to reallocate a few frequencies.