A family member is traveling to a conference and will need to use the convention hall's wifi on both a macOS Sierra laptop and an Android 6.0 Marshmallow phone for web browsing and WhatsApp. How can I best prepare her devices for securely using the conference's wifi network?
A VPN solution would be suitable here. It allows your family member to connect to any public (in your example conference) wifi hotspot and have all their internet traffic protected from being snooped on or otherwise interfered with.
You mention in the comments at the top about trust. Yes a VPN moves the who you trust to someone else. But then that applies to anything on the internet, including which ISP you trust to connect to the internet to start with. If you use an OpenVPN solution you could setup a VPN host at your home and get them to connect to that. That way they will be using your internet. But personally if you do not know how to do that I would suggest using one of the many VPN providers on the internet, some free some paid for.
Also make sure their device has been patched with an AV and firewall enabled to stop local traffic attempting to access their laptop.
There are multiple attack vectors here.
Sniffing the traffic coming from your machine (MITM) in order to steal sensitive information (credentials, credit cards...). A VPN would add a layer of encryption and while your traffic could be intercepted, it would not be decrypted (so would not be of any use for a malicious person)
Compromising your laptop or devices in order to gain access to it. Turning on a host firewall and blocking all incoming traffic would help. Configuring a firewall does require to understand which traffic is essential and should not be blocked, and which one can be blocked. Using security softwares on the endpoint would also reduce the risk of exploring a vulnerability on your machine.
A good way to secure your devices is to have some form of antivirus and malware protection on both Android and Mac. Make sure when downloading anything, to confirm that the link is safe or better yet, during the duration of conference if not necessary just to not downloading anything at all.
This website has some pretty good advice on how to be safe while using essentially public wifi: