If someone is davening shachris at a beis ha’avel where there is no tachnun being said but needed to leave after sh’mona esrei (to use the restroom) before chazaras hashatz, and then goes to a regular minyan to complete his davening, does he need to say tachnun with the current tzibur or since he already completed the primary part of his tefillah with the other minyan that wasn’t saying tachnun he is therefore not required to say it?
This is an interesting question. On one hand the reason we don’t say tachanun in a beis ha’avel is because the time of saying tachanun arouses din, and we don’t want to do this in a bais ha’avel. In your case, that you are no longer in the beis ha’avel you should say tachanun. On the other hand tachanun is a part of shemona esrei, and since at the time you davened shemona esrei you were in the beis haavel, so the davening is without tachanun. The poskim say that the latter understanding is correct and therefore you would not say tachanun.
M:B 131-20, Horav Y. Cahen shlit”a, Poskim.