What do you mean by unlicensed?
But, amoxicillin should never be dosed as that. You need to have a constant serum level above the MIC for B-Lactams to be effective; time-dependent kill.
If pt. was almost anuric, I think it'd be more along the lines of 50-100 mg q8h, you lower the dose, never extend the interval. For B-lactams anyway.
The post-coital thing is interesting, might be right. Should be TMP/SMX though.