I want you to know that what is now, at 3, is not what will be in 2, or 5 or 10 years time. Hold on to that. Look for as many good things about him as you can and praise him and love him for those.
DD1 was a wild toddler. It was hard taking her anywhere because she was the one who would grab the toys from other children, with as much force as she needed, would tantrum round the shops, run away from me at any excuse, ignore most of what I said. I was constantly being called into preschool about her behaviour, usually because her imaginary dogs were biting the boys. It was a nightmare taking her out anywhere so usually I stayed at home.
DD2 was much like your friend's little girl. She was an absolute angel child. At last I knew it wasn't me! She slept, she had manners, she did everything she was asked, she talked politely to adults. She was the easiest preschooler you could possibly imagine.
Once DD1 went to school she gradually settled. Somehow she went from strength to strength. She is 16 now and is a lovely teenager. She has just sailed through her older childhood and teenage years and we have a brilliant relationship. All the friends who warned me she would never settle at school, never do anything, be so aggressive she would end up expelled or worse - they were wrong.
DD2 is now nearly 13, and much harder work. At about 11 she was so challenging I was often posting on teens for advice, in despair. She is getting easier, but still often stroppy and oppositional. I've had to sort out all kinds of issues I never had to even think about with DD1.
So your friend with the angel DD may well have the harder time in years to come!
He is only 3. It's very young and he will change a lot as he grows up. Encourage the positive as much as you can. He will have some lovely qualities, try to bring those out as he grows. And hold on to the possibility that one day you'll be the one with the child you can be proud of :)