From what you said, it seems your friend also loves this guy right? or more correctly, your friend has also developed some feelings for him?
1. Tell your friend to forget thinking about Marriage.
How can your friend think about marriage with someone who she just chats on whatsapp or social media? Can the friendship leads to marriage? Yes it can lead to marriage but that is a later issue.
2.Is the boy telling the truth? This one no one really knows. Your friend does not know this guy and the guys does not know your friend either. They need to meet and get to know themselves more before they can find out whether they are joking or they are being serious.
3. Can it be true love? I don't know what you mean by true love but Yes it can be true Love. She needs to meet the guy and find out based on his actions not his words whether the guy loves her as he claims.
4.Tell your friend to stop being jealous. Is she in a relationship with the guy? Did she accept the guys love proposal on whatsapp? if she did not or has not formally accepted his proposal, what's her business getting jealous about whatsapp pictures?
Finally, just tell your friend to get real and get to know the guy better. Let her take her mind of marriage. Start by getting to know the guy for who he actually is not what he says he is. Once she gets to know the guy by frequent meetings, she will then be able to know whether the guy loves her as he claims or not. Till they have done that, no one will ever know.
I hope my words were not harsh. If they were, please tell her to forgive me as i did not mean to be.