Your friend is most likely right. Here's an easy test.

Take out the film cartridge and the dark slide. Then, try to look through between the space where the dark slide goes in. Use your finger nails or something sturdy in an effort to create as much space as possible between the slit. If you can see the other side between the slit, then the light seal there is eroded and needs to be replaced.

I got a kit from Ebay for cheap, but you can contact Hasselblad directly for a genuine kit, which is not that expensive AFAIK. The replacement is also quite straight forward.