I like how, from the get go, the author kept at least acknowledging that there is something wrong with this scenario, but people still ignore all of it despite no explanations being provided, and insult almost everyone here who finds this unsatisfactory and contrived. I can see some of the comments are very aggressive, but still, the story itself hints at this not being normal in the slightest. For example:
Chapter 1:
"Are you trying to pass off 17 years without contact with a simple 'Long time no see?"
"I'll at least let you explain yourself before I call the police..."
Here the author is obviously pointing out through the characters that this IS weird, and exes that have had no contact with you don't suddenly show up after 17 years without warning at your door. Even best friends don't show up after that much time, certainly not without warning and certainly not to ask for a favour.
Chapter 2:
"I know I am the one asking this of you, but you don't... Get scammed too often... Do you?"
Here, the author is again obviously pointing out that accepting them in just like that, without asking any basic questions, is akin to be gullible enough that you could get scammed often.
Chapter 3:
All of the reactions from his friend.
"There is no way she would do that..." (Trick him)
Closing it all by saying that even if she was tricking him, he would be cool with it just to see her smile.
Here the author is flat out telling you what the character is thinking: he trusts her to an extent, but he is also fine with being tricked, which explains why he doesn't really ask very reasonable questions (some of which his friend asked to him, again, showing that the author is self-aware).
This makes it painfully obvious that yes, she IS taking advantage of him. While it's of his own doing (so it's hard to feel bad for him), he isn't letting her stay because that's the decent thing to do, he still has emotional hang-ups, and it's not guaranteed he would put up with this if someone else in difficulty (a friend or a stranger) had shown up.
This says nothing of the fact that she apparently has a job, and still hasn't brought up the possibilty of paying him back in any way. This is just bizarre, and is just basic human decency. If you were crashing at someone's house with your sons/daughters for an unspecified amount of time, wouldn't you at least ASK to chip in, first thing? Even close friends would bring it up, let alone someone who has been outside of your life for the last 17 years.
Even I want to call this guy a simp now, just what the hell is this plot?