As a BDD sufferer (comes with intense and horrifying social anxiety), I find I struggle with this very same issue. I worry I am actually offensive to look at and am less worthy that everyone else. (my reality not anyone else's reality) so that right there, helps me get to places when I'm scared to leave the house. These things help:

Do something physically active to calm nervouseness before you leave for an outing as it reduces anxiety somewhat.

Prepare to go a few hours before I need to leave.

Tell myself that there will be good karma at the end of my struggle if I just do one more difficult thing ( reward of cosmetic surgery that I "need" ) Does your partner crave something specific at all?

Don't look directly into peoples eyes ( I can't do this at all ) but rather just look "through" crowds, scenery etc. and find a place to mentally escape to ( fantasy land, daydreaming ) etc.

Here's been the most helpful one of all:

Remember Introverted Feeling (Fi) is focussing on Self emotion, and try and master Extroverted Feeling (Fe), to remember that other people may be in a hurry etc. for their own reasons, and no one actually wants to hurt me. This relieves inner self pressure for dealing with self hatred anxiety and paranoia that no one thinks I am deserving of good things, or even of being able to get by in a crowded side walk etc.

None of this might help at all, but it was what I could think of as I deal with the same misfortune as your partner, and this stuff helps me sometimes.