There were two new children in dd's Y1 class at the start of this term. They fitted in instantly and have made lots of friends.
Ds is in Y5. When he was in Y2 a new girl joined, who could only speak German. She fitted in straight away quickly became on of the most popular children in teh class. When he was in Y3 a new boy joined the class - he was immediately accepted. In Y4 an American boy joined, again instantly accepted. Later on that same year a new girl joined and no problems. This year, at the start of Y5 a Polish boy joined and he has made lots of friends despite not speaking much English. And a girl joined because she was badly bullied at a neighbouring school. Again she has fitted in straight away and is happy.
So I think your friend had an isolated bad experience.
I moved schools many times between teh ages of 5 and 13 as a child, and the only time it was a problem was aged 13. But then that is a difficult age anyway iirc
We haven't changed schools, but ds1 has had several kids join his class over the past 2 years (in Y2 and Y3) and they have fitted in really well and are as much part of the class as the ones who have been there since nursery. The school has a mentor system where the new child is assigned a mentor for the first month (a different child from his/her class each week) and that works v well as it means they get close contact with at least 4 other children and it gives them a foundation for friendship.
Ds1 started at the beginning of P2 and it was absolutely fine. There are children leaving and starting all the time (well maybe one or two a term) and the children just take it in their stride - not seen as a big deal.