I am not a pro with these things but I have several pairs of pearls including one pair of graduated cultured pearls I received on my 16th birthday... now approaching 70 and when my daughter was just a little toddler, she broke the strand and while they are individually knotted...I had to get them restrung.
Depending on how long the necklace is, you could use the pearls from the necklace but if they are not natural pearls and very expensive quality, the re-stringing and the number of pearls needed might cost you more then looking for matching pearls for her.
Again, they may be lovely pearls but re-stringing costs are usually by the inch and then you would have to buy decent (14 or 18 K gold)...backings and have them set adding the cost of the gold settings and design of the earrings. I've been through a lot after my mother passed away and I have six different pearl necklaces ranging from excellent quality to very nice cultured vintage pearls and in my humble opinion, I suggest you match the pearls searching a few jewelry stores and buy a pair for her that is to her liking.
