Hello, and welcome to Apple Support Communities desdemona1013,
We can see you've been using If you can't send or receive messages on your iPhone, iPad, or iPod touch, and are finding that your text messages are not going to one of your contacts. This would be the right guide to use.
You've mentioned that the bubble is green so it's either an SMS or MMS message. For these messages, you'd want to contact your carrier. They'd be able to work with you to narrow down what might be happening if these messages are not being delivered.
You've also mentioned this contact does have an iPhone. Have them check their device to check Settings > Messages > iMessage > YOU CAN RECEIVE IMESSAGES TO AND FROM. They'd want to make sure that their phone number is selected in this section. This is mentioned in the section "If you see a green message bubble". We wanted to call out this section specifically since you'd need to check both devices.
We hope this helps!