An AC repair company has told my 84 year old mother that she needs a new UV light on her AC unit to keep the drain line from clogging up and leaking. She just had a clog in the line which the service guy cleared but he is now trying to sell her a $900 UV light system... with a life time warranty.
I do not think this is necessary for the efficient operation of the unit nor for the preventing of the drain line becoming clogged in the future. Should she agree to spending this kind of money on this?

No, make sure the drain hole is kept open.

An AC repair company has told my 84 year old mother that she needs a new UV light on her AC unit to keep the drain line from clogging up and leaking. She just had a clog in the line which the service guy cleared but he is now trying to sell her a $900 UV light system ...with a life time warranty.
I do not think this is necessary for the efficient operation of the unit nor for the preventing of the drain line becoming clogged in the future. Should she agree to spending this kind of money on this?

No tell her to save her money.

The UV light will not keep your condensate drain from cloging up. Its made to keep mold off the evaporator coil. Some of the newer ones are made to kill airborne bactira and oders. They are a good thing to have if you have a breathing problem, but they all produce ozone witch in turn can cause a breathing problem if you have asthma.

No, UV lights are just used to clean the air and its not unnecessary to install if not required.