T = team mates
E = Enemies
R = Team mate with reach
Obviously T E T set up has the enemy flanked but lets assume you have a party member with a reach weapon and they position themselves like this.
RT E T
Does the reach party member still have the enemy flanked?
Yes. The reach wielder has an ally in the space opposite the enemy who threatens.
https://www.d20pfsrd.com/gamemastering/combat/#TOC-Flanking
The image shows #3, Ogre (with reach) and Goblin are flanking the teams Sorcerer.