Is EGR valve change necessary?

Nobody here can tell if your EGR value is irreparable damaged or not. Some valve models are frail and easily damaged, others are quite robust and cleanable until the damage is too big.

Most of the time the failure is that the valve is full of carbon and the actuator cannot move the vale. Some models are quite robust and just seize, but have a good probability that they work again after a thorough cleaning. Other models fail irreparable early.

Diesel cars that see no regular long drive on high load are especially prone to blocked EGR valves. The problem gets worse on prolonged oil changing cycles.

Now the average mechanic has a hard time explaining to you that he must periodically remove a certain item (perhaps on an hard to reach place), clean it, reinstall it and hope that the actuator is still fine.

It is much easier to say "expensive-thingy-broken-must-replace", especially if the cost of replacement (100% success rate) is comparable or less to the cost of labor with the risk of non-success involved.

The glow light flashing and a reduction in power is a classic symptom of an EGR valve problem on VW group diesel engines, so it sounds likely that the mechanic made the right call on replacing it, especially as your car is working again. If your friends are saying it wasn't an EGR valve they are probably wrong.

If your friends are saying that the valve could have been repaired instead of replaced then they could be right. Sometimes EGR valves get clogged up with soot and can be cleaned instead of replaced, however it may not be the best option for you. The biggest cost with the replacement is the labor, and the difference in cost between replacing the part and repairing it isn't that much. A new part is much more likely to work flawlessly, a mechanic could repair the valve and test it only to find once it's back on the car there was an undetected issue. It's better to put on a guaranteed part than one that could still be faulty.