My cousin recently got engaged and now they meet each other in absence of muharram. They have dinner together. I know it is not allowed. My question to you is that is it allowed for me to eat the things e.g chocolates they give to each other. Is it better for me to throw them away because I can not openly tell her that I cannot eat them. Similarly can I eat the sweets which are distributed when someone gets engaged.

No problem. However, if you really care for her, you must tell her that what she is doing is prohibited.

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