My father is 53 years old and his front bottom tooth fell out today, although there does not look anything wrong with the tooth - it hasn't rotted at all. He was diagnosed with a brain tumour last year and is due for surgery on it next week - could the tooth loss be something to do with that?

And is a dentist able to do anything about it for him, should he go visit a dentist?

The tooth may have fallen out due to bone loss (gum disease). At this point it's best to talk to your dad's specialist before scheduling dental appointments.