Is my laughing and saying "Astaghfirullah" after my colleague asked me to "Pray Dhuhr with me" considered apostasy that expels one from the religion, necessitating the pronouncement of the Two Testimonies and performing ghusl, or is repentance sufficient?

No one is permitted to pray on behalf of another, nor is it permissible to request such a thing. This statement is objectionable. However, it does not appear to be a disparagement of religion or a mockery of prayer, so the person who said it does not exit the fold of Islam, but repentance for it is obligatory. As for the questioner, she repudiated the statement by seeking forgiveness, so nothing is required of her.