Every time I send a text to a specific person it makes send the text as a “text message” instead of an iMessage

So every time I send a text to a specific person it makes send the text as a “text message” instead of an imessage. This person had an android before they got their apple, but they have been on their apple for a while. This only happens when I start the conversation, if i am responding to them, the texts are blue. This is just starting to happen recently and I would like to know how to fix it. Can anyone help me?

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