Spider mites (red spider mite) is usually a glasshouse pest. I don't think that in 30 years of professional horticulture, ive ever seen them outside.

RSM are usually the result of hot, dry conditions....they detest humidity and damp. I just keep my conservatory plants sprayed over with tepid water....particularly on the underside of the leaves and try to maintain a good humidity.

Same with house plants.

My dahlias get spider mite and they are grown outside.
I spray with dynamec. If used when you first see them and every 10 days or so it works. You just need to remortgage your house to buy it.
And for other insect nasties I use Decis.
Both are expensive but you don't need to use much in your sprayer so they work out a lot cheaper than the stuff bought at garden centres that is nowhere near as effective as the above.

Google will give you suppliers for both.

Decis is a synthetic Pyrethroid and as such is lethal to Bees so do be careful using it.

Thank you all very much for your advice. I will pass on the information to my friend. ❀