17.5YO DD is allowed to have her (18YO) BF stay over and sometimes stays over at his. Nice lad, nice family. DD and BF have just left school and it's the summer holidays here in Scotland.
This morning I woke up late 
to her reminding me that I had an appointment in 45 minutes and telling me that the BF "is here". Last night he had gone on a planned lads' birthday night out in town after which, as planned they walked the 5(ish) miles home. BF lives in a different town from the other lads and us and we live about 1.5 miles closer to town than the birthday boy and the rest of the lads.
So, he had phoned DD about half-past 12 at night to ask her if he could come back to ours instead of walking the extra 1.5 miles. An hour and a half later he phoned again (waking her) to say he was nearly here - so she could let him in, I suppose. Neither DH nor I heard him - we weren't disturbed.
This afternoon I dropped them and a friend at a country park a few miles away. Later DD texted to say she was walking home. The three of them walked back to a shop beside the friend's house where BF's mum picked them up in her car and drove DD home. (BF lives approx 8 miles from us. Public transport is limited. BF sometimes get the use of his mum's car.) I went out and thanked her for driving DD home, to which BF's mum replied 'No bother on their way to collect something from the birthday boy's home anyway - unrelated to the birthday night out'. Which sort of led me on to mention to her that BF had slipped in to our house in the middle of the night after the night out. (BF's mum didn't know this).
If you were BF's mum how would you have reacted to being told this? If you were me, how would you have reacted this morning? MN opinions sought!