I don't think your friend necessarily needed to leave anything for your son.It was a benefit to him that you both stayed and looked after his house and garden.But I can understand why you may be feeling a bit miffed that your friend didn't offer to pay for anything.Essentially it was a free holiday for her.Yes it was in the summer,but is this a pattern of behaviour,that you are more generous than her,and it's happened before and that's why it still irks now ?
Could it be that 3 weeks highlighted this .It does seem rather mean to have 3 weeks of having everything provided meals,all the organisation even if invited and not even be bought a lunch.For several years a friend always coming to my house,never reciprocated to hers - there is a reason I now know.Dashing to get a table in a cafe whilst I got the drinks and letting me pay most times.etc.A good friend in many ways but really stingy in others.I haven't said anything outright but have arranged far less with her,and our friendship is on the wane.Sad but sometimes it happens.
Hold onto the memory of a lovely summer - especially on an icy day like today.Make a mental note ,but not a grudge,and either accept you will be the organiser and cook if there is a next time and you want company.