With my husband, I found the art was to stop using clothing that had to go on and off over the head. He started to become worried and frustrated by it and I could see he was becoming more reluctant to wear certain things or to face the frustration that getting dressed caused him, with or without help.
Out went jumpers, sweatshirts and t-shirts and in came button-up shirts (long and short-sleeved) and a variety of zip-up or button-up cardigans. If your friend's wife seems like she is flustered by over-the-head clothes, it might be worth him looking at some button-up alternatives for her (if she is still able to partly dress herself, velcro fastenings might help her feel more in control).
Mostly my husband would let me dress him in one go, but the trick was not to rush him at all. If he sensed any hint of being rushed he would become more flustered. Some days he just seemed to struggle more than others, so I would clothe him in what I could to try and keep him warm - but sometimes ended up adding socks or a cardigan to his ensemble as the day went on!
I also found that it was best to avoid any kind of appointments or plans in the mornings. It meant that I sometimes felt much of the day has been wasted - but it did mean I had a better chance of getting him to places in a more relaxed and happy state.
The link
@karaokePete provided goes into a lot of other helpful tips, which I hope helps your friend and his wife.