Sounds just like my issue. Mine was a Merc Alpha One. As long as I held pressure on the shift linkage (bolted on the engine), it would run in forward. If I let go, it would pop out of gear.

Mine worked great until it didn't.... No warning.

It could be cable stretch and just need an adjustment. If it went into gear and stayed there for the trip in, he might get lucky with an adjustment. I wasn't so lucky.