Hi all, when a male client of mine does the lateral lunge, he is unable to straighten the right leg when lunging to the left.

Jennifer,
After reading the above comments, I really think Karin, and Aleks bring up some great pointers to think about.
I would like to add the following suggestion(s).
Have you done a lying hamstring test? and/or does your client have any limited ROM while trying to get into knee extension?
I would do the following, firstly regress the exercise to a lunge in the saggital plane (check for any imbalances). Then I would do some glute and abductor exercises to stabilize and strengthen the affected site. Then try going back into the lateral lunge.
One more thing that just came to mind, is your client properly warmed-up?

All the best,
Shlomo