I had the same issue for me and my daughter. You have to go in on 'your network' and change your wife's entry to manage from assist. She should then get an email saying that someone (you) has asked to manage her tickets. She has to reply then to that email confirming she is OK with that. Then you can go on and buy for her.
One thing to bear in mind too, is that the passwords have probably expired (mine had) so you both may need to go in and rest them.
When I tried it, a pop up came explaining how to do it. It was a bit fiddly, and it is useful to have your wife with you when you do it, but a simple soul like me managed to do it.
Hope this helps, and let me know how you get on