Oh, it was way more than that. A couple of years back, Betty was treating Glen as if he were a male friend her own age, or rather, as if she were his age; like they were buddies. At one point he ran away from home, and Betty found him hiding in Sally's outdoor playhouse; he'd been living there for several days. She didn't tell anyone where she found him, and she let him hang out at the house all day, watching TV, eating, and wearing some of Don's clothes while she washed his. She finally took him home late in the afternoon, and his mother was absolutely LIVID -- she made it very clear that she was suspicious of their relationship, and Betty was warned to stay away from Glen.
I don't think that Betty really realized what a monster she had created (Among other things, Glen asked for a lock of Betty's hair, and she gave it to him), but Glen developed a serious obsession with her, and his mother (Helen Bishop) knew it. I'm guessing that he's furious that Betty married someone else when she got divorced, so now he's transferring the obsession to Sally, who looks so much like her mom. Whatever else, the one thing that you can bet is that this subplot is going to bring out Ugly Betty.
PS: I think that the return of Freddy is meant partly as contrast. Don is beginning to look very like Freddy did in the old days; passing out every night. I think that the situation with Allison is that he knows that he did something in regard to her that he should not have, but does not remember exactly what it was -- I don't think that he remembers the sex. He was buttoned up when he passed out on the couch, so he remembers the business of the keys, but not much beyond that. There was no trace of her in the flat when he woke up -- not like he found a brassiere on the floor or something like that.
