The 350 Mag MPI is new enough it will be tough to find a good used one- here are a couple of ideas though.....
Marine Engines Inc. advertises a new, in crate, GM Marine 350 Gen IE Vortec 325 hp with roller cam for $2859. Not sure if equipped with accessories or not.
Repro-Power Center in Florida has turnkey 5.7L 350 Mag (300hp)MPI complete with all accessories for $4450. They also have access to long and short blocks- call for pricing. All new from Mercury- they are a wholesaler.
Derebery in Texas has them for $3750 exchange- I think they sell low hour/used motors as well.
I would definitely check out places like Doller Offshore and Marc at Bam Marine for pricing as well- negotiate the frieght costs from any of these in the deal.- Hope your friend is up and running soon!- just takes $$$.