Yes. If I had absolute omniscient knowledge of the person's soul I could probably answer your question. But I'm not God, so I can't. In general, I don't think it would be good to deliberately miss Mass. I wouldn't advise someone to do it. But since it is impossible know the mind of God, I'd hesitate to subsitute my judgement. I can't know if that person has sanctifying Grace or not.
You do not need to be omniscient to lnmow what your church teaches...according to your church it is a mortal sin is that not correct? (Or is your church wrong?)
The requirement to attend Mass on Sunday and other holy days of obligation, rooted in the Third Commandment and codified in Church law (cf. Code of Canon Law, canons 1246-48) is a serious obligation for all Catholics. A Catholic who (a) is able to attend Sunday Mass (i.e., who is not impeded by illness, lack of transportation, etc.), (b) knows the seriousness of this requirement, and (c) nonetheless freely chooses to miss Mass, thereby commits a mortal sin (cf. Catechism, no. 2181)