Just to start, as someone who is active in the Autism community I take exception to false statements such as this: "Depending on their condition, some autistic kids are devoid of emotion or reading emotions and showing affection." People with autism are not "devoid of emotion." If anything they feel emotions MORE intensely than other people and can have trouble expressing and reading that emotion. It's a widely believed fallacy that people with Autism aren't capable of emotion or that they don't have emotions and it's simply not true. Ever.
When it comes to birthdays and gift ideas, adopted children and children with Autisms are exactly like any other child. You gift to them based on their likes and trends. The best way to figure that out is to ask his mom directly what he's into lately and ask what size clothing he wears. You might find some clothes in a color or character or theme he likes and in a nice, soft, comfortable texture. You might find a learning toy that would fit in with his likes. You can't go wrong with checking with his mother. She'll appreciate it.
As for the hugs... don't assume that she's jealous or envious that he's hugging you. My daughter is not a hugger or kisser, but she knows that sometimes it's expected. She mainly knows that it's expected by Old People, but most importantly that I will never force it from her and make it clear to everyone that I won't force it or chastise her if she refuses as long as she makes a polite greeting. If she chooses to hug someone in greeting or farewell, I'm proud of her. If she takes a liking to someone in particular (which does happen) it brings tears to my eyes because while she's affectionate in her own way, seeing it in little "typical" ways is amazing to me.
If you're worried that it bothers her, she's your FRIEND. Ask her straight out if it bothers her and if she says yes then ask her how she'd like you to approach it when he wants to hug you. Again, she'll appreciate you being open.