Without knowing much about OpportunityCard's plans for Ethereum integration, this is what I have come up with.
What I imagine is that at the bottom of each opportunity card, if the card owner wants to receive Ether, they add "ETH: (eth address)". Either above or below the "Request Connection" part, a form asking for an amount of Ether to send appears (click an article on thewritersblock.tech to see what I'm talking about.)
If the person viewing the opportunity card is on desktop, all they have to do is have MetaMask installed and they can easily send any amount of Ether instantly.
However, since this is such a new space, mobile support is lacking. Mobile users will have to install a specialized browser that allows interaction with Dapps. Some examples are Toshi, Cipher Browser, and TrustWallet. Until MetaMask is released on mobile, mobile users will have to open the card in a dapp-compatible mobile browser and import their MetaMask wallet details. It's a one-time process, but intimidating for many users nevertheless. It's kind of impossible to make this easy and user-friendly unless the users already use these, which most people don't.
Many people have Coinbase, so one possible solution for mobile could be a button to copy the address to your clipboard and open it in the Coinbase app. I don't know if this is possible though -- I'd have to look into it.
^ EDIT: The Coinbase solution is possible with HTML copy to clipboard and Deeplinks