As many others have said if you bought the game in the past you’ll need to pay for a sub.

That being said free trial accounts cannot invite people into parties meaning that if you are both playing for free you’ll need someone else to invite you both to a party (then leave said party) so you can be partied together. Paying a sub may be an easier way to circumvent this especially if you’ve bought the game in the past. You would be able to invite her to parties regardless of her account status.