It can help, but it might not help.
It all hinges on who else in your extended family has tested. Ancestry is the best for UK testers as it has the largest British database - sites like My Heritage who also offer DNA testing have a primarily North American database.
Some families just don't test their DNA. I know through traditional genealogy that I have a huge family network on the paternal side - grandad was one of 11, his father was one of 13 and so on. Loads and loads of second and third cousins. Yet I have hardly any strong matches because none of them have tested. DH on the other hand has Irish ancestry and has many more matches.
When you get your matches you have to understand what the numbers mean. Ancestry or any other site cannot tell you "this is your cousin" all they can say is that you share a certain percentage of DNA with this other person, and that percentage means they could be a hlaf sibling or a cousin, or an aunt/uncle. You still need to go through the whole process of building trees and working out where you fit in.
Especially with an unknown birth family this is tricky. If it's an unknown father you can usually weed out the matches which belong to your mother - so anything else has to be from the father's side. When both parents are unknown, as with an adoption or abandoned baby scenario, this is an awful lot more tricky and you will really need help. (Unless you strike very lucky and come up with a whole sibling match).
Proceed with caution. Keep speaking to the counsellors and prepare yourself anf your mum for all scenarios. Sometimes these stories have a very happy ending. Other times the birth family doesn't want to know, or people have died, or it turns out the pregnancy was as a result of rape or something. Your mum has to work through all these "what ifs" first before rushing out and doing a DNA test.