Are the vehicles paid for or does he owe on them?
BMW: Big Maintenance Worry. Get rid of it. Sell it for whatever he can get and use the money to mitigate the cost of another vehicle or repair of the truck.
What's the condition of the rest of the truck? There are a couple issues here to consider.
If he owes, then he's got the cost of repairs on top of what he owes. Also, he may end up repairing the truck for a pretty penny and then end up having the transmission fall out or something. Can't ignore that possibility.
If the truck is otherwise in good condition, then an engine replacement isn't much more than a decent used car of similar age. If he sells the BMW, then he could mitigate the cost of truck repair significantly. If it costs $5,000 to replace the engine and he gets $1,000 or $1,500 for the BMW as-is, then the truck is only a $3,500 to $4,000 repair.
I don't have any experience with such an engine, but based on other's postings here, it's in-for-a-penny, in-for-a-pound. Either replace the engine (long block would be my preference), or risk a very real possibility of "cheaper repair" costing more in the long run.