I don't recall any credible poster on this forum recommend any kind of stretch or strengthening (including flexbar) while TE pain is active. The near unanimous advice is to rest until pain free, then slowly start PT to regain strength, and STOP immediately if pain returns.
My personal experience was that I couldn't do any kind of movement against resistance without the pain returning. All my elbow could handle at first was isometric resistance (no moving), gradually building up to some movement, and finally being able to use a flexbar pain free. It took me 6 months to fully recover to where I'm playing tennis again pain free.
Remember with TE, any pain is no gain (actually moving backwards).