Would you guys consider a 13 year age gap (between a 31 year old male and an 18 year old female) too big of a difference?

Just to clarify: someone I know has genuinely fallen for a man of that age, and a lot of my friends find it strange and alarming. Just wanted to hear other people's thoughts on this.
Would you guys consider a 13 year age gap (between a 31 year old male and an 18 year old female) too big of a difference?
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