My DS1 was just like this at this age. He now (at age 7) has a dx of ASD (High functioning) and is almost impossible to handle.
I have 2 younger NT DSs and they are 'normal boys' in that, for instance, they have tantrums occassionally, when tired or thwarted. Their tantrums don't last for 2 or 3 hours, though.
They have interests and are heavily 'into' certain things (eg DS2 who looooves Playmobil), but not like DS1, who gets totally engrosed in subjects to the extent that he learns everything there is to learn , and then will suddenly drop that subject, and move onto another.
DS1 also has a need to spend alot of time outdoors, but in natural environments. A place with lots of people or things going on will make him very anxious.
I could go on and on...It is only when you experience the intensity of these things in a child with ASD that you realise what is meant by an 'obsession' or 'sensory processing difficulty'. It is completely different to what you see in 'normal' children and when the child has all these things at once, it's obvious to those close to the child that there's some sort of a problem.
I think if people have no experience of children with ASD they can hear the list of things like the OP gave in her opening post and think it's an exaggeration, or that she's 'over thinking', or else these things only happen occassionally or in certain circumstances.
One thing that really upset me when I was considering whether or not something was 'wrong' was hosts of people telling me that their child did this or that, or it's just 'normal' behaviour. I would find myself giving more and more examples of his extreem behaviour in order to try and convince the person I was talking to that this was NOT 'normal', and make myself feel and sound like all ever did was complain about the boy.
My Ds1's Godmother is one of the few 'friends' I had at the time who never once said I was worrying over nothing, or 'Oh yes, mine had tantrums/obsessions/refused to wear clothes at that age'. God bless her, she just listened, poured the tea and just kept telling me she loved him no matter what, and whatever the Dx, he was, and always will be, what he is. And we have to love him for what he is.
I still appreciate her input and her opinion. The opinions of the 'friends' who tried to fob me off are no longer saught. Some are well and truly OFF my Christmas list!
When I was in the same position as your friend, I welcomed my good friend broaching the subject gently and giving me the chance to discuss all my concerns without judging me or dismissing my worries. It really is the most exhausting, confusing and soul detroying thing to be living with a child with challenging behaviour, no matter how well you deal with it.
One last thought, I have dealt with it so well that I am currently fighting tooth and nail to have some sort of help from one of the many agencies 'involved'. All of them say the same thing- that we seem to be managing very well, and then they run away. Some have even openly admitted that if we weren't 'managing' they'd be able to offer all sorts of programmes to help his behaviour. The fact that I'm totally on my knees and my other 2 Dss never get a look in is not seen to be an issue!