Try and make the bolt holes go thru a stiff area ,not just a random part of the thin floor section,also remind him to inform his insurer to keep it legal if a passenger was hurt.
When I did mine I put 2 of the passenger double seats inline with the sliding door.
I used two steel plates about 300mm wide and about 500mm long to sit over the corrugations in the van floor.
Seats placed on top and bolt holes marked.
Some careful shuffling will see all the bolt holes sit above the lower section of corrugations.
Marked and drilled holes and welded m10 weld nuts to the sheets.
Bolts the plates to the sheets and place back in van.
You can then tack the sheets in correct place before removing seats and fully welding the plates in.
I did it this way because of the shear amount of rust under a transit and I need to occasionally remove seats. This way all fasteners are internal and dont seize up.
For straight bolting in, the minibus rails, that are approved are fitted with m8 high tensile bolts, every 75-100mm ish along the rail.
Two rails per seat. Minimum 500mm rail.
50mm washers on the underside.
Personally I dont like them as the deform the floor when tightend
And dont forget your seat belt anchors
I'm pretty sure i read somewhere that if its a triple seat the two outer seat belts must be fixed to the floor the middle ones ok as a lap belt
I fitted some seats into an older transit for brother in law. They were ten seats and had built in belts and came from a bus in a local scrapyard. I had to weld two legs to one end as they had been bolted to the side of the bus. I just stood them in place and drilled through the floor then put spreader plates on the under side with a nut welded on.
My LT35 has unwin rail and seats thats attach to it with seat belts on the seats, The seats can be removed for extra space and i have some anchor straps that go into the unwin rail to stop anything sliding.