You don't need to split the PNR. Each passenger has a separate ticket. A PNR may have multiple tickets and it sounds as though that is what you have done.

Read the fare rules carefully. Chances are that they will say something such as a $300 penalty for making a change prior to departure and $0 value if a no show. If that is the case, then cancel close to departure. Your friend will still have $100 (in this example) to use within the year.

But, if for some reason, the flight is cancelled or significantly delayed, she may cancel the ticket for a full refund to the original form of payment which sounds like your credit card.

Just a future suggestion as this is a public board, no matter how close a friend, don't book tickets for others. You can help them out by giving them booking information or they can provide your their credit card details. But, in this case it may only be $400, but it could be a lot more - OP is out the cash.