Technically, zakat is an Islamic obligation, therefore it is done out of necessity rather than choice. If a person does it by choice, it is defined as sadaqah. Your non-muslim friends are doing it because they want to, not because they have to, so it's considered sadaqah. I think Zakat is done in a specific way and with specific intent.
Zakat in the Qur’anic verse “and whatever you give in charity, desiring Allah’s pleasure it is these (persons) that shall get manifold” covers both obligatory and recommended sadaka (poor–rate). Similarly, the word salat (ritual prayer) includes both prescribed and mustahab (recommended) ritual prayers”23
If one receives a different kind of Zakat-eligible wealth, most scholars say that one starts a new calculation of a year for its Zakat due date. Some say one should pay Zakat on it upon receipt, or add it to one’s Zakat payment if one has an established Zakat accounting year. Same Kinds of Wealth Can Be Added to Zakat Due Dates for EaseThe Zakat (or almsgiving) is considered the third pillar of Islam. The root word of Zakat also means purification. Zakat is a means of purification of one’s material wealth and soul as well. It is a reminder that the person’s wealth is a blessing from Allah. It is a blessing that one ought to share with others. ibfy.