As the prior answer nothing to do with you.
You can write the letter but at the interview the officer even dont look at it, because they take the decission base on their own ties to their country. Strong ties: good jbs , house, cars, banks account etc etc. Ties that show to the officer that they dont go overstay their visa and work here iligally.
Only if they are very old like 80 they hav better chance, because a 80 old person wont come to work here.
Also depends from what country they are.
Poor country harder to get a visa with low income