Straight from southwest.com:
https://www.southwest.com/html/custo...nd_regulations
If the Companion is not traveling with the Member, the Companion will be required to pay for his/her ticket. The Member and the Companion must check in together and, if asked, present the Companion Pass card at the Southwest Airlines' Ticket Counter, Skycap podium, or Departure Gate in order for the Companion to travel. Valid photo identification for both Member and the Companion is required. The Member and the Companion must board with their assigned group(s).
tl:dr
1) if you aren't both travelling, the companion must pay for his/her own ticket
2) You must both board the plane
3) You must both be available to possibly show ID and the Companion Pass
And this is only a small portion of the Companion Pass Ts & Cs