Is it considered Shirk (associating partners with Allah) for a person to say to a girl, "I want to worship your feet," knowing the translation of the phrase and that he did not intend by it associating partners with Allah or actual worship, but rather he intended by it kissing her feet to increase lust?

If your friend merely intended to kiss the foot, his statement is not considered disbelief (kufr) or apostasy. The default is that a Muslim remains Muslim until there is clear evidence to the contrary. Our scholars have said, "If there are ninety-nine reasons indicating the disbelief of a Muslim and one reason for him to remain Muslim, then the mufti and the judge should act upon that one reason." His repentance is accepted, especially if he performs some expiations for sins, such as 'Umrah. Allah forgives the polytheist if he embraces Islam and abandons polytheism, as Allah Almighty says: "Say to those who have disbelieved, if they cease, what has passed will be forgiven for them." (Quran 8:38) And His saying, the Almighty: "Except for those who repent, believe and do righteous deeds. For them Allah will change their evil deeds into good ones. And Allah is ever Forgiving, Most Merciful." (Quran 25:70)