As for usefulness, I'm not sure it's true that German is more useful than Spanish for a traveller in Europe. Yes, there are more native German speakers in the continent as a whole, but German speakers are excellent at speaking English accurately, fluently and with a very good accent. So a young Russian who is proficient in English would have no trouble at all navigating his way around Germany using only his English.  However, he would have a much harder job doing the same thing in Spain. Proportionally, fewer Spanish speakers speak English well, and they all struggle far more with pronunciation than Germans do. This young man may understand Kate Winslet perfectly, but he might find speaking English to Spaniards somewhat more challenging.

I'm also not convinced about German being a useful lingua franca in Europe, either.  Thirty or more years ago, this was true, but not any more. In a country such as Hungary, for example, it seems to be only the older people who learnt German. The younger generation all learn English. If he's lucky enough to already have a decent grasp of English, then German may not give him much of an advantage.

I vote for Spanish.

 If he just needs to take up another language for the sake of meeting the needs of the school curriculum, then Spanish is probably the language that will "disturb" his studies the least.

German would involve learning a lot of complex grammar involving agreements of pronouns, articles, and adjective endings for three genders and four cases. Admittedly, as a Russian he would have a head start with the concept of cases, but learning them would still take up quite a bit of time. Word order is strict. Vocabulary is another challenge with German. While many simple everyday words are similar to English, the more complex vocabulary is very different. (The opposite is true of Romance languages.)

For someone who has the time and commitment to learn a European language, French is a very useful skill which is definitely worth acquiring. But it isn't the easiest option for this young man.  French has a lot of potentially confusing homophones, and a lot of words have silent endings. As these endings often carry the grammatical meaning, ( for example, the difference between an infinitive and a regular past participle) learners who try to study French in a casual manner really struggle to relate what they hear to what they read.

With Spanish, on the other hand, what you see is what you get. There are only five vowel sounds  ( compared to 20 or so in English!) and these correspond to the five alphabetical vowels. There's a clear relationship between written and spoken Spanish, meaning that, once you've learnt a few basic rules and the differences between English and Spanish  ( eg the pronunciation of 'j' ),  if you hear something you can write it and if you read it you can say it. Word classes are also more easily identifiable - for example, it's clearer to identify a Spanish past or present participle when you hear it than the equivalent French one.