I have done this trip three times, all from San Diego, once in late April, and twice in early December. On one of the December trips, we had one slightly rough day where two currents meet, either right before or right after Cabo (forget which). This is fairly common certain times of year, but we only experienced it that once. I have heard that the first day out of SD or LA and the last day back can be rough, but that was not our experience. However, some of our friends who went the week after us in December had a very rough day the last day of the cruise.
I thoroughly recommend the electronic seasick relief band for your friends. It is pricey (about $139), but in my opinion, well worth the price. I have been fine on all of my nine cruises, one with a very rough day in the Caribbean. I even dared to go on a cruise around Cape Horn and to Antarctica with it, but I must admit the waters were unexpectedly smooth. I use it on car trips also.