My son is also a 'grumpy napper' and wakes up from nap very unhappy. For him, the key points are to go to him as fast as possible when he wakes up, and respond in a happy, cheerful way even if he's starting to tantrum, and then as soon as possible engage him back into the day. This usually means afternoon snack or going outside to play, but any favorite activity may work.
Once he enters the post-nap grouchy mood, no amount of comforting him seems to help, and he will just cry and cry and escalate into a tantrum, but pushing through and getting on it with it in a reassuring and cheerful way helps him return to a better mood.
An alternate thing to try is to time his naps so that he's awake when his mom leaves. Some kid are confused, disoriented and distressed when a parent 'just disappears' while they sleep, and can rationalize and cope with the separation more if they have a chance to hear mom or dad say goodbye and remind them of the time they will be back (it helps to use routine-centric language like "I'll be back after lunchtime" or "I'll be back before you take your bath"-- anything that they understand to happen on a predictable cadence).