Dear questioner,
Thank you for your important question.
May Allah, Most High, reward you for your desire to increase in knowledge.
Sending text messages to your friend’s sister to plan a surprise for him is permissible as long as nothing prohibited is being discussed.
If you begin to feel romantic feelings in the process of these messages, you must stop messaging and pursue the proper method of asking for a girl’s hand in marriage.
In general, messaging between genders should always be within daylight hours. Adding a third person to the discussion is always a good choice and a sign of piety.
Please see Principles of Gender Interaction.
And Allah alone knows best,
Omar Popal
Checked and Approved by Shaykh Faraz Rabbani