I'd say there's normally no warranty on repairs, you have to know your clients and make good judgements sometimes and allow some flexibility imo. Old dentures can and do break, nothing lasts forever. You also state the tooth "fell out" so that looks like a freebie based on technical workmanship error.
I think if it is a repair due to some pre-exisiting condition, like loss of vertical in a removable appliance resulting in tooth breakage, you are not liable and should let the doc know this will probably be a problem going further for the patient. The tooth didn't fall out, did it? If so, technical error.
Dentures break for a reason whether it be due to a clinical fault or a technical fault. IMO repairs should not be guaranteed but on the same token, i like to keep my clients. If a repair that i have done breaks again, i will charge for it regardless of fault. It is the responsibility of the clinician to find the cause of the fault if it is a clinical error
If I made the denture I warranty it for 5 years, and I have never regretted it. Anybody else's denture gets a warranty until it goes out the door.(especially and old denture)