Leak stoppers depend entirely on the SIZE of leak.
In other words, the leak must be teeeeny-weeenie to begin with.
The system must be able to hold a decent vacuum (no gross leaks)
and the solution, if you wanna call it that, circulates through
the system, and at a leak, combining with 'air', it solidifies.
Most available are for 134A refrigerant, since that's what's used
in the automotive industry, AND, it's an easy sell to someone
sweltering in his car on the way to work.....
I personally don't know (just no reason to look) any leak stoppers
for household or industrial a/c, but i expect they're out there.
One of the other snags, is that the evaporator section is under
vacuum when in operation, so instead of leaking 'out',
it's leaking 'in'....hence, air in system, until shut down,
and pressure builds up in system.....
but at that point, the damage has already been done, by the leaking 'in'.
If the leak is stopped at that point explained above,
then further evac and recharge would be necessary at some time later.
So, as far as your friend is concerned, I would say that if you/he
found something you wanted to bet on, it wouldn't hurt to take a
shot, as long as the material is compatible with the refrigerant
used, and also something that the serviceman is comfortable with
injecting into the system.
He's not gonna want any responsibility for the system 'going south'
as a result of the 'stuff' he puts in.
And, the owner must realize more than one recharge might be necessary.
If there was anything 'that good', you would think he would
have suggested it, no???
Shoot, EVERYBODY would use it !!
So, it's a krapshoot
As far as the units being rented 'below the note',
that's a whole nuther discussion, and if that's the way he's gonna run,
he needs to be in another business.