I suspect that if you do select a seat she will be allocated next to you, and if you don't, well she will be allocated next to you ... See what happens if you select a seat (I think you can deselect later) ... go into select a seat (with an empty seat beside it) and then go into make a new dummy booking to see if you can allocate the seat beside you or it is no longer available. If the latter, there's a good chance it has been blocked.

Although with SQ IT systems/web interface, one can never be sure if it is malice or imcompetence that precludes things such as this.