This must have been tough for Billy & Frank. According to this Blabbermouth article. ZZTop band member Dusty Hill wanted them to continue, and with his guitar tech, Elwood.
https://www.blabbermouth.net/news/watch-zz-top-play-first-show-following-dusty-hills-death/?fbclid=IwAR0ARbLEX4JMge54vt7vKRleqKJwYpyRU14mGD9x9YYT3IAXZKQIpvWcHbE