There are many canons adressing membership in a Catholic Church sui iuris. The basic rule for a baptized infant (<14) is that they are of the Church sui iuris of the Catholic parent or guardian, or of the Catholic father, or of the Catholic mother (if both Catholic parents agree or there is not a Catholic father), regardless of the particular initiation ritual used. There are some other rules related to change at marriage and also of children of parents who change.
So it sounds like your friend never received approval for a change of ritual church to the Ruthenian Catholic Church from the Latin Church. So the prescripts of the Latin church should have been followed. This is consistent with the CCEO eastern canons 40 and 403 quoted before, as the Latin Church is also a Church sui iuris.
Yes, conversion is addressed by a different set of canons than change of ritual church for Catholics. Since a requested change of ritual church of a Catholic may be denied by one of the two bishops, it is not guaranteed.
It is up to the individual to follow the norms of their proper ritual church. If one does not will to do that, then the hierarchs and presbyters cannot force one to. And, no one can licitly induce one to change ritual church. Legally, one is a member of a Catholic Church sui iuris only based upon the baptism or by an approved change of ritual church (which can happen by election at marriage).
The canons in the 1983 CCEO are based in part on the expressions made at Vatican II.
Sacrosanctum concilium (Vatican II, 1963):
"14. Mother Church earnestly desires that all the faithful should be led to that fully conscious, and active participation in liturgical celebrations which is demanded by the very nature of the liturgy. Such participation by the Christian people as “a chosen race, a royal priesthood, a holy nation, a redeemed people (1 Pet. 2:9; cf. 2:4-5), is their right and duty by reason of their baptism.”
Decree on the Eastern Catholic Churches, Orientalium ecclesiarum (1964):
“6. All members of the Eastern Rite should know and be convinced that they can and should always preserve their legitimate liturgical rite and their established way of life, and that these may not be altered except to obtain for themselves an organic improvement. All these, then, must be observed by the members of the Eastern rites themselves. Besides, they should attain to on ever greater knowledge and a more exact use of them, and, if in their regard they have fallen short owing to contingencies of times and persons, they should take steps to return to their ancestral traditions.”