The spouse, under both natural and civil law, is automatically his spouse's legal guardian.
The question was, "does XXXX have a right to impose *HIS* wishes....", not, who the GUARDIAN is. (And I stated NATURAL rights so there is no question or confusion w/made-up "rights" which are really privileges.) And there is no such thing as a "natural law" regarding "guardians".
There is NO NATURAL right to kill ANYone (and indeed, I'm assuming here that MS brought up the "Terri wanted to die" for his convenience, not cuz she actually said it). Murder is a violation of natural rights, regardless of your relationship.