Hi,
You've answered your own question really.
It could be a loose connection or it can be an after effect of the water damage.
I wouldn't say he should repair it for free unless he gave you warranty that it wouldn't have any further issues.
If I was him (as a repairer myself) would look at it again for free as you've already been back once.
See what the diagnosis is. The video card board on their could have after damage markings so it may need a new replacement board, Really depends how bad the water damage was on the inside.