Missing Mass without a good reason IS a mortal sin.
Your friend in RCIA was told correctly if they were told that missing Mass because of things beyond their control (such as being sick, being a caregiver when no one else is available, a work schedule that cannot be changed) is NOT a mortal sin - but you should get clarification from your priest.
If I’m truly sick, I’m going to stay home rather than infect my entire parish by going to Mass. I am a paramedic, and there has been more than one occasion when I worked a 24-hour shift on Saturday (can’t go to Mass) and then be held over on Sunday because someone called in sick (can’t go to Mass), or I have been obligated to work during the day on both Saturday and Sunday (7am-7pm) and can’t find coverage for a couple of hours so I can go to Mass. In those situations, I talk to my priest about a dispensation. What can he say? That I should call in for one of those days, risking my job? That I should quit my job altogether, even though jobs in the medical field are also somewhat difficult to come by?