Be VERY careful here. I used to work in insurance and theres so many clauses and restrictions regarding driving other vehicles.

Some may only allow you to drive other cars of a lower insurance group than the one you have insured. Theres also many other 'get outs' that the insurers often put in the policy schedules, to avoid paying out.

Best to check with your insurers and try and get something in writing if possible.

As PT said though, usually 3rd party only so im sure he wouldnt let you drive his ferrari anyway.