I don't know the answer but just want to say I'm sorry for your loss and how nice it is to have such a great reminder of her! Do you know what color you are going to paint it??

Go to the paint department of Lowe's, Home Depot Ace Hardware, etc. and ask for outdoor paint. Try Sherwin Williams or Benjamin Moore if you have those near.

Although the colors are limited, some of the spray paints advertise that they are for outdoors:
Valspar outdoors

If you can keep the rain off it by putting it on a covered porch, for example, it will last longer.

satine, is it then a wooden step stool? I had bought a couple of little wooden bistro chairs (inexpensive) and stained them green. Then I piled on a few coats of spar (marine) varnish. It's not the first time I've bought something formulated for boats for outdoors here, but this is the rainy pacific northwest and we need all the help we can get. I do put the chairs away in winter but they still look good at about 10 yrs old now. Folding, and handy...I should have picked up four and not just two.

Whatever you do, I know you'll enjoy the repurposed 'plant stand'.

I use Rustoleum 2x for a lot of my outside garden things.

I think an exterior opaque stain will last longer on wood outdoors than an exterior paint

Sherwin Williams Duration Exterior. If the wood is never submersed, duration will last 8 years or more.

Thanks for all the responses. Looks like I have some options. I don't have any covered area for the stepstool but will bring it indoors after the summer season. I don't know what color I'll use but something on the vibrant side--not my usual but my friend was an artist and all her paintings used vibrant colors.

We use Rustoleum spray paints here too. Not sure about the 2X and too lazy to go out to the garage, but we recently got rid of a couple of painted 18 year old Adirondack chairs that had rotted away inside but the paint looked pristine :-) I bumped one and sawdust fell out.