You should not plan to date your friend's current partner.
Eric and Donna have been in a relationship for a few years.
Steven wants to ask Donna out for dinner.
Steven invites Donna to dinner and makes it clear it is just a friendly outing.
Steven sends a flirty text to Donna inviting her out to dinner.
Donna agrees to go to dinner with Steven and has an enjoyable night.
Donna is surprised by the text message and rejects Steven's advances.