Overstay is a chargeable offence..even if for one day...those idiots who say just pay the fine, nothing will happen..please remember that your overstay immigration record with Thailand will be forever , they can anytime banned you from entering or obtaining a visa.
I've seen some Thai girls together with the foreigner and she too had a Thai bf..I'm sure the Thai bf also knows she been sleeping around yet he can tolerate the gf sleeping with other men..😅