Alec was excited about the party and not sparing any thought to what it may or may not mean. He didn't want to judge Milo's mom - especially not after that blow-up Milo had had ages ago - and he didn't think she'd be self-absorbed enough to snatch him away so close to the end of the school year, so this, to him, was just a happy visit.
Xander, on the other hand, was far more worried. He focused his attention entirely on putting together several small tarts for the party. He didn't know how many more people Milo might invite, though he doubted there would be many, but the size had already doubled just from what he knew, from two to four. Plus, he couldn't go without taking something for the grands. He was very familiar with the complicated relationship of a son who loved a very flawed mother, but as he was Milo's friend, not his mom's, he was highly biased and he knew it. What if she had finally convinced the grands he could go live with her early? What if she was taking him away now whether the grands liked it or not? If that was what was best for Milo, that'd be one thing, but yanking him out of school with only a couple of months to go? How was that good for anyone? As it was, he still felt uneasy about it all. In his eyes, while she did seem to mean well, it was all about her and her guilt and her mistakes. He was self-aware enough to know he was probably overdramatizing it, but he didn't want Milo to get hurt. Outside of Sloan, he was the only close friend Xander had. He counted Peter among his friends, of course, and he had pals at Judo, but Milo was a close Friend. It was selfish, but he didn't want to lose his friend, and if he had to, he wanted assurance that Milo would be taken care of and his needs put first.