Is it permissible to drink non-dairy milk at a meat meal? For example, there is a common custom in our community to serve almond milk at engagement parties, and it often happens that people eat meat hors d'oeuvres with one hand while holding a cup of almond milk in the other. Although there is, strictly speaking, no prohibition against partaking of almond milk with meat, doing so gives the appearance of eating meat with cow's milk, and thus should perhaps be forbidden due to Mar'it Ha'ayin, the prohibition against acting in a manner that may be misconstrued as forbidden conduct.
A similar question arises at catered affairs where a meat meal is served and margarine is placed on the table. Since the margarine resembles butter, eating meat with margarine on the table gives the appearance of eating meat with butter, and thus should perhaps be forbidden on the grounds of Mar'it Ha'ayin.
The Halacha in these cases would depend upon a debate among the Halachic authorities concerning the parameters of the Mar'it Ha'ayin prohibition. The Rama (Rabbi Moshe Isserles, Poland, 1525-1572), in his glosses to the Shulchan Aruch (Yoreh Dei'a 87), ruled that this law applies only when the given activity appears to transgress a Torah violation. If, however, a person acts in a way that may be misconstrued as a Rabbinically forbidden act, he does not transgress the law of Mar'it Ha'ayin. According to his view, it would be permissible to hold a glass of almond milk while eating meat, or have margarine on the table during a meat meal. The Torah prohibition of Basar Be'chalav (meat with milk) applies to cooking or eating meat together with milk. Eating meat and milk one after the other, or eating one while the other is on the table, is forbidden only by force of Rabbinic enactment, and not on the level of Torah law. Hence, in the cases described above, the individual may be suspected at most of transgressing a Rabbinic prohibition, and thus according to the Rama, one may hold a cup of almond milk while eating milk or keep margarine on the table during a meat meal.
Many other authorities, however, including the Taz ("Turei Zahav" by Rabbi David Halevi, 1586-1667) and the Shach (Rabbi Shabtai Ha'kohen, 1623-1663), disagreed with the Rama, and applied the rule of Mari't Ha'ayin in all situations where one might be suspected of forbidden activity. Even if one will be suspected of committing an act permitted by the Torah but proscribed by the Sages, he must, according to these authorities, refrain from the given activity. Hence, according to these views, serving non-dairy milk or margarine with meat would be forbidden unless some clear indication is made that the milk or margarine is in fact not dairy. For example, one may place pieces of almond in the almond drink to make it clear that this is not cow's milk. And when serving margarine at a meat meal, one should leave the packaging on the table so that everybody will see that it is not butter.
It should be noted that this applies only to situations where non-dairy milk is served together with meat. Mar'it Ha'ayin would not apply when parve ice cream or non-dairy creamer is served for dessert, after the meat has been cleared from the table. Since the meat is no longer at the table, eating parve ice cream or drinking non-dairy milk does not give the appearance of a violation at all, and therefore no prohibition of Mar'it Ha'ayin is entailed. In such a case, one need not make an indication that the ice cream or creamer is not dairy. (Yabia Omer, Helek6, Yore Dea Simon 8)
Summary: If one eats or serves meat together with non-dairy milk or margarine, he must clearly indicate that the milk or margarine is not dairy, such as by leaving the packaging on the table, so that nobody will suspect him of partaking of meat with milk. One may, however, partake of non-dairy milk, creamer or ice cream for dessert, after the meat has been removed from the table, and he need not make an indication that the milk, creamer or ice cream is parve.